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Assessing the parameterisation of the settling flux in a depth-integrated model of the fate of decaying and sinking particles, with application to fecal bacteria in the Scheldt Estuary
Deleersnijder, E.; de Brauwere, A. (2010). Assessing the parameterisation of the settling flux in a depth-integrated model of the fate of decaying and sinking particles, with application to fecal bacteria in the Scheldt Estuary, in: Deleersnijder, E. et al. (Ed.) (2010). Tracer and timescale methods for understanding complex geophysical and environmental fluid flows. Environmental Fluid Mechanics, 10(Spec. Issue 1-2): pp. 157-175
In: Deleersnijder, E. et al. (Ed.) (2010). Tracer and timescale methods for understanding complex geophysical and environmental fluid flows. Environmental Fluid Mechanics, 10(Spec. Issue 1-2). Springer: Dordrecht. 1-295 pp., more
In: Environmental Fluid Mechanics. Springer: Dordrecht. ISSN 1567-7419, more
Peer reviewed article

Also published as
  • Deleersnijder, E.; de Brauwere, A. (2010). Assessing the parameterisation of the settling flux in a depth-integrated model of the fate of decaying and sinking particles, with application to fecal bacteria in the Scheldt Estuary. Environ. Fluid Mech. 10(1-2): 157-175. dx.doi.org/10.1007/s10652-009-9151-6, more

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Keywords
    Marine; Brackish water
Author keywords
    Settling; Mixing; Mortality; Decay; Pycnocline; Residence time; Depth-integrated model; Reactive tracer

Authors  Top 
  • Deleersnijder, E., more
  • de Brauwere, A., more

Abstract
    The fate of reactive tracers is often modelled by depth-averaged equations. When integrating the depth-resolved equations, it appears that the term describing the settling of particles is dependent on the concentration just above the bottom. Because in a depth-averaged framework this quantity is not available, the settling term needs to be parameterised. The most natural choice is to make the settling flux dependent on the average concentration. This approximation is acceptable if the water column is well mixed, but these conditions are not necessarily met in real applications. Therefore, this study aims at assessing and understanding the error made by using a depth-averaged model in a range of realistic conditions. For the definition of these conditions, typical values for the Scheldt Estuary and the Dutch-Belgian coast were taken. The realistic inspiration for the reactive tracer in this study is the fecal bacterium Escherichia coli, whose own dynamics are characterised by settling and gradual decay by mortality. In an attempt to understand the relative importance of several factors like settling, mortality, mixing and stratification on the error made by a depth-averaged approach, a number of simplified test cases were investigated. It follows that, as expected, the error is acceptable if the situation is mixing-dominated. However, the effect of mortality and stratification was less obvious in advance. For instance, it appeared that errors can also be significant if settling and mortality have the same characteristic timescales. Stratification often has the effect to increase the error made by the depth-averaged model.

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